The present invention relates to a solid electrolyte type fuel cell and, more particularly, to a solid electrolyte fuel cell which is of a planar type structure and is obtained by using a low-resistant solid electrolyte stable over an extended period of time, a heat-resistant part obtained by covering the surface of a heat-resistant alloy with an electrically conductive film resistant to oxidation and reduction, and the like.
Referring to FIG. 13, there is shown the basic structure of a solid electrolyte fuel cell in which a cathode (e.g., perovskite-type La.sub.0.9 Sr.sub.0.1 MnO.sub.3) 32 and an anode (e.g., NiO/ZrO.sub.2) 33 are provided on both sides of a solid electrolyte 31 (e.g., partially stabilized zirconia) in the form of films, and oxygen (air) is supplied to the cathode 32 as an oxidizing agent, while hydrogen, carbon monoxide, naphtha gas, naphtha-modified gas, liquefied natural gas (LNG), LPG or the like is fed to the anode 33 as a fuel gas. Zirconia (ZrO.sub.2) shows an electrical conductivity of 0.5 .OMEGA..sup.-1 cm.sup.-1 at 1,000.degree. C. due to ion conduction, i.e., the transfer of O.sup.2-, and is stabilized by calcium or yttrium because it is very fragile. The cell reactions occurring in such an example are expressed in terms of: EQU Cathode: 4e.sup.- +O.sub.2 .fwdarw.20.sup.2- EQU Anode: O.sup.2- +H.sub.2 .fwdarw.H.sub.2 O+2e.sup.-
with O.sup.2- being transferred through zirconia.
The foregoing refers to the structure of a unit cell. In order to connect a plurality of integrated unit cells in parallel (or in series), connections are made between the electrodes of the adjacent unit cells by means of interconnectors. The structure of an actual fuel cell is then determined depending upon how to integrate together the unit cells. Until now, some integrated structures have been proposed and are now under development so as to put them to practical use.
With respect to the structure of this type of fuel cell, Westinghouse Electric Corp. and Electronics Techniques Laboratories have proposed such a cylindrical structure as illustrated in FIG. 14.
Referring to FIG. 14, a fuel gas is fed into a tubular member 41 and an oxidizing gas is passed thereover. As will be appreciated from an enlarged view of a wall region included in the same figure, an integrated cell structure comprising a porous anode 43, an interconnector 44, an electrolyte 45, a composite oxide cathode 46 and an airtight film 47 of alumina is formed on a porous alumina wall 42 defining a support tube.
Besides, another cylindrical structure and interconnection structure have been proposed by Westinghouse Electric Corp. and such a monolithic type structure as shown in FIG. 15 by Argonne National Institutions. In FIG. 15, reference numeral 51 stands for a solid electrolyte, 52 an anode, 53 a cathode, 54 an interconnector, 55 a fuel passage and 56 an air passage.
In the cylindrical structure shown in FIG. 14, however, the current flows through the cathode-solid electrolyte-anode in the radial direction of the cylindrical body but, in an integrated assembly, the current generated in the unit cell flows through the cathode-interconnector-anode in the lengthwise direction of the cylindrical body. Consequently, the current is required to flow through a long current path running through the cathode and anode of high sheet resistance, during which increased ohmic losses occur. This is because there may be no choice but to use a material of increased area resistance as an electrode material due to a serious electrode corrosion problem caused by the operation of the solid electrolyte type of fuel cell at high temperatures. With the solid electrolyte type of fuel cell of another cylindrical structure proposed by Westinghouse Electrical Corp., such an ohmic loss problem caused by a long current path running through the anode and cathode of increased area resistance cannot still be solved.
Turning to the monolithic type of fuel cell according to Argonne National Institutions, shown in FIG. 15, it achieves as high degrees of integration and short current paths as cannot be obtained with the cylindrical type. However, it poses a reliability problem due to its complicated structure and the need for sophisticated constructional techniques, and is thus far from practical use.
In recent years, on the other hand, cells of new structures have been proposed with a view to developing fuel cells of higher performance.
Referring to FIG. 16, there is shown a honeycomb structure 61 formed of a solid electrolyte such as the partially stabilized zirconia, wherein fuel 62 and air (oxygen) 63 are alternately fed to cells in countercurrent relationship, and an anode is formed on the wall surface of the cell to which the fuel 62 is fed, while a cathode is formed on the wall surface of the cell to which the air (oxygen) 63 is supplied.
Referring to FIG. 17, fuel 74 and air (oxygen) 75 are fed to every other space of stratified spaces 72 and 73 defined by a plurality of solid electrolyte partitions 71 in the perpendicular directions, and an anode 76 is formed on the fuel (74) side of each partition 71 while a cathode 77 is formed on the air (oxygen) side.
The cells of the types shown in in FIGS. 16 and 17 are expected to give an extremely high energy density per unit volume and be suitable for mass-production since conventional ceramic techniques are applicable thereto.
Incidentally, stabilized zirconia used as the solid electrolyte for fuel cells show much superior electrical properties as expressed in terms of low resistance, but is mechanically so fragile. For that reason, particular attention is now paid to partially stabilized zirconia which has higher resistance value but possesses higher mechanical strength, as compared with the stabilized zirconia.
Referring then to the partially stabilized zirconia, it is present in the form of a mixture of a tetragonal phase (hereinafter referred to as the T-phase) with a monoclinic phase (hereinafter called the M-phase), and has the property of lowering its electrical conductivity in the presence of the M-phase. As the M-phase-free zirconia consisting of the T-phase alone is held at high temperatures over an extended period of time, the T to M-phase transformation occurs, resulting in an increase in the proportion of the M-phase. Because the density of the M-phase is lower than that of the T-phase, when the T to M-phase transformation occurs, the volume of grains is so increased that intergranular fracture proceeds further, resulting in further increased resistance value and decreased mechanical strength.
Thus, the problems with the partially stabilized zirconia are that when it is used as a solid electrolyte, the proportion of the M-phase is increased by sintering at a temperature as high as 1,400.degree. to 1,500.degree. C. or upon used as a part of fuel cells at high temperatures over an extended period of time, leading to a lowering of electrical conductivity, while intergranular fracture proceeds, resulting in increased resistance value and decreased mechanical strength.
Preferably, the interconnector for the solid electrolyte type of fuel cell should meet the following requisites.
(1) It should be stable in an oxidizing and reducing atmosphere at high temperatures.
(2) It should be an electrically good conductor in an oxidizing and reducing atmosphere at high temperatures.
(3) It should possess a coefficient of thermal expansion close to that of an oxide ion conductive solid such as stabilized zirconia.
(4) It should have a coefficient of thermal expansion close to that of an electrode material.
The cathode collector should also satisfy the following requisites.
(1) It should be stable in an oxidizing atmosphere at high temperatures.
(2) It should be an elctrically good conductor in an oxidizing atmosphere at high temperatures.
(3) It should possess a coefficient of thermal expansion close to that of an oxide ion conductive solid such as stabilized zirconia.
(4) It should have a coefficient of thermal expansion close to that of an electrode material.
Heretofore, metals or electrically conductive ceramics have been used for interconnectors and collectors. However, when metallic interconnectors and collectors are used at temperatures of 600.degree. C. or higher, there are formed on their surfaces oxides, which then give rise to considerably increased contact resistance and hence increased power losses due to resistance, thus resulting in deteriorations of fuel cell characteristics. As the electrically conductive ceramics to meet the above requisites, proposed are composite metal oxides, e.g., perovskite-type oxides expressed by La.sub.1-x M.sup.1.sub.x M.sup.2 O.sub.3 (wherein M.sup.1 is Sr, Ca or Ba and M.sup.2 is Co, Fe, Mn, Ni or Cr), esp., La.sub.1-x Sr.sub.x CrO.sub.3. However, although such ceramics are electrically conductive, they possess nonnegligible resistance. In the cylindrical thin-film type of fuel cell proposed by Westinghouse Electrical Corp., wherein the perovskiter-type oxide is used as the cathodic material, the cathodic resistance accounts for about 65% of the total cell resistance, and forms a barrier against improvements in the energy efficiency of fuel cells. The most serious problems with the fuel cells of the types shown in FIGS. 16 and 17 are posed by the collector in the structure of FIG. 16 and the inerconnectors in the structure of FIG. 17, respectively.